@meyap Do a test.
In Windows Explorer (File Explorer), browse to "C:\Users\yourusername" and create a new folder named "Opera Downloads".
Then, make a test standalone installation of Opera to a folder named "Opera Downloads Test" on your desktop.
Then, in that test installation, goto the URL opera://settings/downloads and set your download folder to that that "Opera Downloads" folder.
Then, for a while, download things in just that Opera and test if it shows any incomplete downloads you encounter and test how long the list at the URL opera://downloads gets. And, test if resuming works for any incomplete downloads.
If an incomplete download doesn't allow you to resume (option), we would need to know the URL for that specific download to check if the server serving the download supports resuming or not.
After a while of testing, you should know if something is wrong with your regular Opera installation or not.